Determination of Analgesic Equipotent Doses of Inhaled Metoxyflurane vs. Intravenous Fentanyl

Traumatized patients frequently experience pain prehospital due to the fear of adverse cardiovascular effects (e.g. hypotension) and respiration depression of opioid treatment, leading to insufficient analgesia. Therefore, there is of interest to investigate other analgesics that do not have these adverse effects.

In Norway, methoxyflurane has been approved for the emergency relief of moderate to severe trauma pain in conscious adult patients. Metoxyflurane (Penthrox®) do not have these adverse effects (hypotension and respiration depression). It is easy to administrate via an inhalator, and is therefore suitable for use prehospital. This would possible help to treat pain better prehospital.

There exists no data of how effective metoxyflurane is as an analgesic compared to an opioid. The aim of this study is to determinate the equipotent doses of inhaled metoxyflurane vs. intravenous fentanyl.

Ten healthy volunteers (18 - 64 years) will be enrolled in a randomized, double blind, placebo-controlled, crossover study using a standard experimental pain model: CPT- Cold pressor test (ice water). Our group has used this experimental pain model in earlier studies and a crossover study is suitable to compare different drugs.

Possible side effects will be recorded for both metoxyflurane and fentanyl: sedation, dizziness, itching, nausea and vomiting. Respiration frequency and non-invasive blood pressure will be recorded every as possible low blood pressure or respiration depression can occur.

The aim of this study is to determinate equipotent doses of inhaled metoxyflurane vs. intravenous fentanyl. We have chosen to compare with fentanyl (the comparator) because it is a well-known drug in clinical practice for acute pain treatment. In addition, both drugs have a fast onset (minutes) and the analgesic effect last for approximately the same time interval (20-30 min). Therefore, they are comparable in a clinical setting.

There exists no exact data of equipotent doses between metoxyflurane and fentanyl, only retrospective clinical data. One retrospective study compared intranasal fentanyl with inhaled metoxyflurane for visceral pain prehospital in 1024 patients. The initial dose of fentanyl was 0.018 mg and the total mean dose at hospital arrival was 0.036 mg. In the metoxyflurane group 51.9% received 3 ml, a second dose was used in 41.9% and 6.2% received a third dose. Metoxyflurane produced the greatest initial pain scores reduction, and intranasal fentanyl provided greater pain reduction by hospital arrival.

From these data we have chosen two doses of fentanyl to be compared with 3 ml Penthrox: 0.025 mg and 0.05 mg intravenous. Both drugs will also be compared to placebo (NaCl 9 mg/ml).

Inclusion criteria

• Healthy volunteers
• Age 18 - 64
• Both sex
• No chronic disease
• No regular medication
• Recruited from the general population
• Signed informed consent and expected cooperation of the subjects for the treatment and follow up must be obtained and documented according to ICH GCP, and national/local regulations

Exclusion criteria

• Use of pain medication the last 2 days before a session
• Use of complementary medicine the last 2 days before a session
• Use of regular medication
• Previous substance abuse
• Pregnancy
• Know allergies or serious side effects to opioids or metoxyflurane
• Use of alcohol last 24 h before each session
• Exclusion criteria with respect to fentanyl:
• Hypersensitivity opposite the active substance (fentanyl) or other opioids
• Hypersensitivity opposite the excipients (to fentanyl): sodiumclorid, water for injection, hydrochloric acid or sodium hydroxide
• Respiratory depression without artificial ventilation
• Use of MAO-inhibitor or use of MAO-inhibitor by two weeks before inclusion
• Elevated intracranial pressure or brain trauma
• Hypovolemia or hypotension
• Myasthenia gravis
• Exclusion criteria with respect to metoxyflurane:
• Use metoxyflurane as anesthetic
• Hypersensitivity opposite metoxyflurane or fluorinated anesthetics
• Hypersensitivity opposite the excipient (to metoxyflurane):

Butylhydroksytoulen

• Malignant hyperthermia or persons with suspect genetic predisposition for malignant hyperthermia
• History (to the volunteer or family) of serious adverse effects after administration of inhalation anesthetics
• Volunteers showing sign of liver damage after use of metoxyflurane or halogenated anesthetics
• History of liver disease
• Clinical significant reduced kidney function or history of kidney disease
• Changed of level of consciousness of any cause, including brain trauma, drugs or alcohol
• Clinical detected cardiovascular unstability
• Clinical detected respiratory depression